Definition
Lumbar puncture (spinal tap) is the insertion of a hollow needle into the subarachnoid space of the lumbar cistern, typically at the L3/L4 or L4/L5 intervertebral space, to obtain cerebrospinal fluid (CSF) for diagnostic or therapeutic purposes.
Key Landmark
The intercristal line (Tuffier's line) connects the highest points of both iliac crests and passes through the L4 spinous process or L4/L5 interspace. In adults, the spinal cord terminates at the conus medullaris (L1-L2), making L3/L4 and below safe for needle entry.
Layers Traversed (Superficial → Deep)
Diagnostic Indications
- ● Suspected meningitis (bacterial, viral, TB, fungal)
- ● Subarachnoid haemorrhage (xanthochromia after 12h)
- ● CNS malignancies (leukaemia, lymphoma, carcinomatous meningitis)
- ● Demyelinating diseases (MS — oligoclonal bands)
- ● Guillain-Barré syndrome (albuminocytologic dissociation)
- ● Measurement of CSF opening pressure (idiopathic intracranial hypertension)
- ● Neurosyphilis, Cryptococcal meningitis
- ● Normal pressure hydrocephalus (tap test)
Therapeutic Indications
- ● Intrathecal chemotherapy (methotrexate, cytarabine)
- ● Intrathecal antibiotics (amphotericin B)
- ● Spinal anaesthesia
- ● CSF drainage in idiopathic intracranial hypertension
- ● Myelography (contrast injection)
Absolute Contraindications
- ✗ Raised ICP with mass effect (risk of cerebral herniation)
- ✗ Infection at the puncture site (cellulitis, abscess)
- ✗ Spinal epidural abscess at LP site
Relative Contraindications
- ⚠ Coagulopathy (INR > 1.5, Platelets < 50,000/μL)
- ⚠ Therapeutic anticoagulation (hold warfarin/heparin)
- ⚠ Spinal deformity or prior lumbar surgery
- ⚠ Uncooperative patient (consider sedation)
Relevant Anatomy
CSF Characteristics (Normal)
- • Appearance: Clear, colourless (like water)
- • Opening pressure: 6–20 cm H₂O (lateral decubitus)
- • Volume: 120–150 mL total (adults)
- • Production: ~500 mL/day by choroid plexus
- • Turnover: ~3–4 times/day
Normal CSF Values
- • WBC: 0–5 cells/μL (lymphocytes)
- • RBC: 0 cells/μL
- • Protein: 15–45 mg/dL
- • Glucose: 50–80 mg/dL (⅔ serum glucose)
- • Gram stain: No organisms
Equipment Required (LP Tray)
Sterile Supplies
- • Sterile gloves & gown
- • Sterile drapes (fenestrated)
- • Antiseptic (Chlorhexidine 2% or Povidone-iodine)
- • Gauze swabs
Anaesthesia
- • Lidocaine 1–2% (3–5 mL)
- • 25G needle (skin wheal)
- • 22G needle (deeper infiltration)
- • 5 mL syringe
Spinal Needles
- • 22G Quincke (cutting) — standard
- • 25G Whitacre (pencil-point) — less PDPH
- • 22G Sprotte (pencil-point)
- • Length: 3.5" (adult), 1.5" (paediatric)
Collection
- • Manometer with 3-way stopcock
- • 4 sterile collection tubes (numbered)
- • Tube 1: Chemistry/Serology
- • Tube 2: Microbiology (Gram stain, C&S)
- • Tube 3: Cell count (least traumatic)
- • Tube 4: Special (cytology, PCR)
Additional
- • Adhesive bandage (post-procedure)
- • Sharps container
- • Pulse oximeter & BP monitor
- • Resuscitation trolley nearby
Pre-Procedure Checks
- • Informed written consent
- • Check coagulation (INR, Platelets)
- • CT Brain if indicated
- • Baseline vitals
- • Time-out / WHO checklist
Step-by-Step Technique
Standard midline approach in lateral decubitus position
Patient Positioning
Lateral decubitus (preferred for pressure measurement): Patient lies on their side, knees drawn to chest, chin tucked, back at edge of bed, spine perpendicular to bed. Shoulders and hips stacked vertically. OR Sitting position: Patient sits on bed edge, leaning forward over a pillow on a table. Useful in obese patients but cannot measure opening pressure accurately.
Identify Landmarks
Palpate both iliac crests → draw imaginary intercristal line → identifies L4 spinous process. Palpate the interspinous space at L3/L4 or L4/L5. Mark with skin indentation or marker.
Aseptic Preparation
Hand hygiene → don sterile gown and gloves. Clean skin in concentric circles (centre outward) with Chlorhexidine 2% (preferred) or Povidone-iodine. Allow to dry (minimum 2 minutes). Apply sterile fenestrated drape.
Local Anaesthesia
Raise skin wheal with 25G needle using 1–2% Lidocaine (1–2 mL). Infiltrate deeper tissues along planned needle path with 22G needle (additional 2–3 mL). Wait 2–3 minutes for full anaesthetic effect.
Needle Insertion
Insert spinal needle (bevel parallel to dural fibres / pointing cephalad in lateral position) in the midline, directed slightly cephalad (toward umbilicus). Advance slowly through each tissue layer. A distinct "pop" or "give" is felt traversing the ligamentum flavum and dura (~4–5 cm depth in adults).
Confirm Placement & Measure Pressure
Remove stylet → CSF should flow freely. If bloody, allow few drops to clear (traumatic tap) or check tube 1 vs tube 4 for decreasing RBC. Attach manometer via 3-way stopcock. Measure opening pressure with patient relaxed (legs slightly extended). Normal: 6–20 cm H₂O. Document opening pressure.
CSF Collection
Collect CSF passively (never aspirate) into 4 numbered sterile tubes. Collect 1–2 mL per tube (total 8–15 mL typical). Send: Tube 1 → Chemistry; Tube 2 → Microbiology; Tube 3 → Cell count; Tube 4 → Cytology/Special. Measure closing pressure if indicated.
Needle Removal & Post-Procedure
Replace stylet before withdrawing needle (reduces incidence of PDPH). Apply gentle pressure and adhesive bandage. Patient to lie flat for 30–60 minutes (though evidence for this is debated). Encourage hydration. Monitor vitals. Document procedure, volumes collected, opening/closing pressures, and CSF appearance.
💡 Clinical Pearls
- • Bevel orientation parallel to longitudinal dural fibres reduces PDPH incidence
- • Use atraumatic (pencil-point) needles to reduce post-dural puncture headache by 50%
- • If bone is hit, withdraw to subcutaneous tissue and redirect — do not simply advance deeper
- • Paramedian approach may be used in elderly patients with calcified interspinous ligaments
- • In obese patients, use longer needles (5") and consider sitting position or ultrasound guidance
CSF Analysis & Interpretation
| Parameter | Normal | Bacterial Meningitis | Viral Meningitis | TB Meningitis |
|---|---|---|---|---|
| Appearance | Clear | Turbid/Purulent | Clear/Slightly hazy | Clear/Fibrin web |
| Opening Pressure | 6–20 cm H₂O | ↑↑ (>30) | Normal/↑ | ↑ |
| WBC (/μL) | 0–5 | 1000–10,000+ | 50–1000 | 50–500 |
| Cell Type | Lymphocytes | Neutrophils (PMNs) | Lymphocytes | Lymphocytes |
| Protein (mg/dL) | 15–45 | ↑↑ (100–500+) | ↑ (50–100) | ↑↑ (100–500) |
| Glucose (mg/dL) | 50–80 (⅔ serum) | ↓↓ (<40, <⅓ serum) | Normal | ↓↓ (<45) |
| Gram Stain/Culture | Negative | Positive (60–90%) | Negative | AFB (10–20%) |
Traumatic Tap vs SAH
- • Traumatic: RBC decrease tube 1→4, clots present, clear supernatant
- • SAH: RBC same all tubes, no clots, xanthochromia (yellow supernatant after centrifugation >12h)
Special Tests
- • Oligoclonal bands: Multiple Sclerosis
- • India Ink / CrAg: Cryptococcus
- • VDRL: Neurosyphilis
- • ADA levels: TB meningitis (>10 IU/L)
- • PCR: HSV, Enterovirus, TB
Complications
Post-Dural Puncture Headache (PDPH) — Most Common (~30% with cutting needles)
Mechanism: CSF leak through dural defect → low CSF pressure → traction on meninges and intracranial vessels
Features: Bilateral frontal/occipital headache, worse upright, relieved by lying flat. Onset 24–48h post-LP. May have nausea, tinnitus, photophobia.
Management: Conservative (bed rest, hydration, caffeine 300–500mg, analgesics). If persistent >48h → Epidural blood patch (10–20 mL autologous blood) — 85–95% effective.
Prevention: Atraumatic needles, smaller gauge, replace stylet before withdrawal, bevel parallel to dural fibres.
Back Pain (13%)
Cerebral Herniation (Rare but Fatal)
Epidural/Subdural Haematoma
Infection (Rare — iatrogenic meningitis)
Nerve Root Irritation / Radiculopathy
Post-Procedure Care
- ● Monitor vitals q15min × 1 hour
- ● Encourage oral fluids (>2L in 24h)
- ● Bed rest 1–4 hours (varies by institution)
- ● Assess for headache, limb weakness, urinary retention
- ● Check puncture site for CSF leak or haematoma
- ● Send samples to laboratory promptly
- ● Document: indication, consent, position, level, needle type/gauge, attempts, opening/closing pressure, CSF appearance, volumes, complications
When to Seek Help
- • Severe/worsening headache not relieved by lying flat
- • New neurological deficits (leg weakness, numbness)
- • Urinary retention or incontinence (cauda equina)
- • Fever post-procedure
- • Persistent CSF leak from puncture site
Paediatric Considerations
Key Differences
- ● Conus ends at L3 in neonates (vs L1-L2 in adults) — perform LP at L4/L5 or L5/S1
- ● Position: Sitting (held by assistant) or lateral decubitus with assistant maintaining flexion
- ● Needle: 22G × 1.5" (neonates) or 22G × 2.5" (older children)
- ● Depth: ~1–1.5 cm in neonates, ~2–3 cm in infants
- ● Do NOT over-flex neck (risk of airway compromise in neonates)
- ● Volume: 1–2 mL total in neonates (max 1 mL/kg in children)
- ● Topical anaesthesia: EMLA cream 60 min prior (or subcutaneous lidocaine)
- ● Monitor SpO₂ continuously during procedure
Normal Paediatric CSF Values
| Parameter | Neonate | Infant/Child |
|---|---|---|
| WBC | 0–30/μL | 0–5/μL |
| Protein | 20–170 mg/dL | 15–45 mg/dL |
| Glucose | 34–119 mg/dL | 40–80 mg/dL |
| Opening Pressure | 8–11 cm H₂O | 10–20 cm H₂O |
⚠ Neonatal Precautions
- • Higher RBC tolerance (up to 500/μL may be normal in term neonates)
- • Higher protein is normal (especially preterm: up to 170 mg/dL)
- • Septic neonates may have normal CSF — always correlate clinically
- • Rule of 2's for suspected bacterial meningitis in neonates: WBC >20, Protein >200, Glucose <20
Common Paediatric Indications
- • Rule out neonatal meningitis/sepsis
- • Fever without source (<28 days)
- • Seizures with suspected infection
- • Febrile seizures (complex/atypical)
- • Irritability with bulging fontanelle
- • Suspected encephalitis
- • Suspected meningitis (headache, neck stiffness, fever)
- • Intrathecal chemo (ALL)
- • Idiopathic intracranial hypertension